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Watery Asteroid

Water vapor appears to be surrounding Ceres, the largest asteroid and one of several dwarf planets in the solar system (artist's concept, left). Sunlight may vaporize some of the ice at its surface, or water may squirt from a subsurface lake or ocean. The Dawn spacecraft is scheduled to arrive at Ceres (shows in a Hubble Space Telescope image, right) in early 2015, so it may be able to resolve the water's origin. [Left: ESA/ATG medialab; Right: NASA/ESA/SWRI/Cornell/Univ. Maryland/STScI]

Ceres is the solar system's largest asteroid — a big ball of rock and ice about 600 miles wide, orbiting the Sun between the orbits of Mars and Jupiter. But astronomers recently discovered that Ceres is probably behaving a bit like a comet: some of its ice is vaporizing in the heat of the Sun.

Comets are balls of ice mixed with rock. When a comet gets close to the Sun, some of its ice vaporizes, surrounding the comet with a cloud of debris. Sunlight and the solar wind push some of this material away from the comet, forming a bright, beautiful tail.

Ceres is a long way from the Sun. But its orbit is a bit lopsided. When Ceres is closest to the Sun it gets a little warmer. Astronomers watched it with Herschel Space Telescope during a recent close approach. They discovered that water vapor was indeed escaping from the big asteroid — just like a comet.

But unlike a comet, the amount of water vapor from Ceres is modest — the equivalent of a couple of gallons of water per second. The water probably is coming from the asteroid’s surface as it’s heated by the Sun. But there’s an outside chance that it may instead come from geysers — jets shooting from reservoirs of liquid water below the surface.

We should find out which solution is correct beginning early next year, when the Dawn spacecraft enters orbit around Ceres. It’ll give us our first close-up view of the solar system’s largest asteroid — which just may be acting like a comet.

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